There are many applications where instrumentation must be placed in a hostile downhole environment, e.g., geothermal energy development, oil field exploration and production, etc. As an example, the geothermal borehole at Fenton Hill, NM, extends to 4270 m with bottom hole temperatures as high as 350.degree. C. and total pressures of 82.7 MPa. Current technology tools and instruments can have difficulties at temperatures above 200.degree. C., i.e., signal quality degradation, calibration drifting, and electrical component failures.
Thermal protection in downhole environments is presently provided by the use of temperature hardened components and/or passive thermal protection systems. A typical passive thermal protection system consists of a hot service dewar and a set of heat sinks to provide for interior low temperature energy storage. Passive heat absorption is volume limited by the heat sink and other material inside the protected space. Further, a passive protection system can reach thermal equilibrium only at the wellbore temperature. It will also be appreciated that the protective dewars are equally effective in confining heat dissipated by the electronics as they are in resisting heat input from the wellbore. There is no passive mechanism for transferring heat from inside the dewar back to the wellbore against the temperature gradient.
Downhole research under increasingly harsher conditions and higher wellbore temperatures indicates a need for a high temperature, multiwatt cooling system capable of greatly increasing the residence time and the reservoir temperature capability of instruments beyond the limits currently available from passive temperature protection systems. There are, however, many restrictions that must be met by any active refrigeration system for downhole use, i.e., the geometries and dimensions already established by the oil industry for downhole casing and tubing, tool standards, and typical transport chutes in logging trucks or trailers. Further, the refrigeration system must be reliable in the downhole environment and enable downhole operations over extended periods of time.
These and other problems associated with downhole environments are considered by the present invention and active refrigeration using an acoustic refrigerator enables reliable downhole cooling of associated electronic packages. Accordingly, it is an object of the present invention to provide a compact active refrigeration system for application to a downhole environment.
Another object of the present invention is to provide a compact acoustic refrigeration system.
One other object of the present invention is to provide an extremely efficient and compact heater and heat exchangers capable of transferring high heat fluxes to and from the acoustic refrigerator elements.
Additional objects, advantages and novel features of the invention will be set forth in part in the description which follows, and in part will become apparent to those skilled in the art upon examination of the following or may be learned by practice of the invention. The objects and advantages of the invention may be realized and attained by means of the instrumentalities and combinations particularly pointed out in the appended claims.